Base on my observation, IP traffic monitors only every IP connect to router with exception of transmission usage. That difference explain why wan bandwidth is always greater. It would be best that this transmission be classified as "others" category. That way, we can easily see if total match up. Hopefully, Teaman will modify his source code.

Interesting observation:
After a few days, it appears like the Subnet totals on IP Traffic DO add up to the same as the bandwidth (or pretty close).

From everything I've read about IP Traffic, I still believe that it should be tracking bandwidth, not total traffic. I'd certainly appreciate anyone else with some experience of the two tolls chiming in.

IMO iptraffic is counting datas between each user from/to other hosts/vlans/wan. If you have a NAS device (in LAN or VLAN) and you will download some files then iptraffic will count a traffic but bw usage will not.